dm: core: Add a note about how livetree updates work

dm: core: Add a note about how livetree updates work

The unflattening algorithm results in a single block of memory being
allocated for the whole tree. When writing new properties, these are
allocated new memory outside that block. When the block is freed, the
allocated properties remain.

Document how this works and the potential memory leak, as well as
mentioning that updating the livetree is actually supported now.

Signed-off-by: Simon Glass <sjg@chromium.org>